With recent progresses of a digital technology in electronic equipment, larger-capacity and nonvolatile semiconductor memory devices have been developed vigorously to store data of music, image, information, and so on. For example, a nonvolatile semiconductor memory device using ferroelectric as a capacitive element is already known in many fields. In addition to the nonvolatile semiconductor memory device using such a ferroelectric capacitor, attention has been focused on a nonvolatile semiconductor memory device (hereinafter referred to as ReRAM) using a material which changes its resistance values by application of electric pulses and retain the changed states, because it is highly compatible with a standard semiconductor process.
As a resistance variable layer, nickel oxide layer (NiO), vanadium oxide layer (V2O5), zinc oxide layer (ZnO), niobium oxide layer (Nb2O5), titanium oxide layer (TiO2), tungsten oxide layer (WO3), cobalt oxide layer (CoO), etc., are used. It is known that such a transition metal oxide layer exhibits specific resistance values when voltages or currents which are not less than thresholds are applied thereto and retain the specific resistance values until new voltages or currents are applied thereto. The transition metal oxide layer has a feature that it can be manufactured using the existing DRAM process steps without modifying it.
Patent literature 1 discloses a configuration of a cross-point ReRAM in which, a memory plug is formed at a cross-point of a X-direction conductive array line and a Y-direction conductive array line. This memory plug is composed of a resistance variable memory element and a diode element having a metal-insulator-metal (MIM) structure. The memory plug comprises seven layers stacked together. A composite metal oxide sandwiched between two electrode layers is a memory element, and the MIM diode element is formed on the memory element.
Patent literature 2 discloses a ReRAM including a substrate provided with two or more bit lines arranged to be spaced apart from each other and in parallel with each other, two or more word lines arranged to be spaced apart from each other and in parallel with each other and crossing the bit lines, respectively, resistive structures formed at cross-points of the bit lines and the word lines and above the bit lines, respectively, and diode structures formed on the resistive structures in contact with the resistive structures and the word lines, respectively, lower electrodes formed on the substrate, resistive structures formed on the lower electrodes, respectively, diode structures formed on the resistive structures, respectively, and upper electrodes formed on the diode structures, respectively.
Patent literature 3 discloses a cross-point ReRAM including a resistance variable element composed of a lower electrode, a variable resistor, and an upper electrode between a bit line and a word line, and a non-linear element connected in series with the resistance variable element.